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Will Depart in...



Challenger 300 FMC CDU

Started by ame, September 21, 2020, 09:39:07 pm

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September 21, 2020, 09:39:07 pm Last Edit: September 24, 2020, 03:41:10 pm by ame Reason: Removed duplicate images
Since I have nothing better to do (!) I am plodding on with an FMC CDU design for a Challenger 300 that I started ages ago. If anyone is interested that's great, but if not that's fine too.

I have the idea that this device should be nothing more than a display and a keyboard. The display plugs into a PC like a monitor, and the keypad plugs in like a keyboard. Thus, all the smarts are elsewhere.

So I have a 5" VGA monitor and driver, off the shelf. And I'll make a PCB with tiny pushbuttons in the right place, hooked up to a PC keyboard driver chip (such as Holtek HT82K629A) or an Arduino Pro Micro running QMK firmware.

The keypad will plug in like a regular keyboard. Keys A-Z and 0-9 will generate those key codes. The 12 function keys (6 left and 6 right) will be F1 to F12, and all the other keys will generate some other key press, probably ! to ) on the top row, above the number keys. No shift, ctrl, or other keys.

I have 3D printed a panel for this, and some actuators for the buttons. The technique I am using could be used for any FMC for any plane. It's still a work in progress, so don't hold your breath. :)

Edited to add: The gray panel is an early prototype, without the rebate for the VGA display module. The white panel is the top 2/3rds of the current design with the VGA rebate. My printer is quite small, so I have to print the panel in four parts and glue them together. I also have to print the actuators, and make sure they move freely. I'm not sure about the decals yet...

Joe Lavery


An interesting project indeed!

I notice you always display your images twice in each post.

Trevor has referenced this issue at the beginning of the forum header.

If you just click upload on your attachments. you will get a single insertion, if you then also click insert you get them twice.

'Life isn't about waiting for the storm to pass, it's about learning to dance in the rain!'


Yes, I wondered about that. The upload has an 'insert' button next to it. It seemed to me that uploading was one operation, then deciding where to put it was another.

I wonder if there is a forum administrator option to show/not show uploaded files next to their associated post.

Trevor Hale

No there isn't but if you click upload, don't click insert.  Its better with the attachments on the bottom of your post regardless, otherwise the forum can look strange.

Trevor Hale


Director of Operations
Worldflight Team USA



September 22, 2020, 04:13:29 pm #4 Last Edit: September 24, 2020, 03:42:35 pm by ame Reason: Acknowledge edit in earlier post.
Roger wilco.

I edited the post to remove the references to the images, so they only show up once. And I edited this post to say that I had edited the other post.


Just a small update. I printed the rest of the frame, and glued it together. I'm pretty happy with the switch openings and actuators, but I need to make a slightly more permanent test assembly.

When I build the 3D model I print out the coordinates of the centres of the switches, which I hope to import into Kicad for the PCB design. Then everything will line up.

The switches are tiny SMD tact switches, 6x6 mm and 3mm tall. The fascia is 6mm tall, so I open a 4mm cavity for the switch body then the actuator has a 0.7mm flange (to stop it going through the aperture) and a 4.3mm body on top (so it sticks out by 2mm). It really is pushing the capabilities of the printer, but it does work (especially after a spot of filing or some sandpaper).

I also hooked up the 5" VGA module. That works too, and looks pretty smart. It is a ZJ050NA-08C display module and KYV-N2 V6 display driver board. These are available as a pair from the usual sources for about $40 or so.


So, on further research the Holtek HT82K629A is probably not going to work for me. It seems to be targeted at a compact keyboard, so F11 and F12 are not directly available, they are only accessible if you hold an extra key down (Fn) first. Much like a laptop keyboard. Some other keys I wanted to use are also not available directly, so I will put that chip to one side.

That means I will probably use an Arduino Pro Micro and build a QMK keyboard. I will get full control of the key matrix, but it will be USB only. I might be able to make a PS/2 version too, but code for that seems to be less common (since, of course, you can just buy an HT82K629A...)

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